1. Field of the Invention
The present invention relates to a method for the preparation of a soft magnetic ferrite powder sinterable at a low temperature and a method for the production of a laminated chip inductor from the soft magnetic ferrite powder produced by the above method.
2. Prior Art
In recent years, technologies for decreasing electronic machines and equipment in size and weight are remarkably developing, and various devices are accordingly being rapidly adapted to surface mounting. As an inductor device in particular, a so-called chip inductor formed by integrating a magnetic material and a coil is used in many cases, and it is desired to improve its performances.
For a chip inductor, generally, an Nixe2x80x94Cuxe2x80x94Zn ferrite which is an oxide magnetic material is used as a magnetic material, and Ag or an Agxe2x80x94Pd alloy is used as a conductive material for a coil.
In the production of a chip inductor, first, starting compounds containing Fe, Ni, Cu and Zn, respectively, are mixed, for example, with a ball mill, then, the mixture is calcined, and a calcined product is pulverized to obtain a soft magnetic ferrite powder. The soft magnetic ferrite powder is kneaded together with a binder and a solvent, to obtain a magnetic material paste. Further, a conductive material powder is kneaded together with a binder and a solvent, to obtain a conductive material paste. Then, these pastes are printed repeatedly to laminate magnetic material layers and conductive material layers, then, the so-prepared laminate is fired, and further, an external electrode is formed, whereby a chip inductor is obtained.
Meanwhile, for achieving high performances of a chip inductor, essentially, the ferrite is sufficiently densified and, further, it causes no reaction with the conductive material. Further, the conductive material is not caused to have failures such as line breaking. As a conductive material, it is preferred to use Ag as a simple substance due to its low electric resistance, while Ag has a melting point of as low as 960xc2x0 C. For achieving high performances without causing any reaction between the conductive material and the ferrite and any line breaking, peeling, etc., of the conductive material, preferably, the firing is carried out at a temperature which is lower than the melting point of Ag and which is as low as possible, e.g., a temperature of 920xc2x0 C. or lower.
However, it has been already pointed out that when an Nixe2x80x94Cuxe2x80x94Zn ferrite powder is fired at a temperature of 920xc2x0 C. or lower, the densification of the ferrite does not well proceed so that it is difficult to obtain a ferrite sintered body excellent in electric properties such as magnetic permeability.
A method for the production of an Nixe2x80x94Cuxe2x80x94Zn ferrite sintered body is disclosed, for example, in JP-A-5-175032 to the present Applicant. The above Publication describes that the ferrite sintered body is improved in density by firing a ferrite material in an atmosphere having a lower oxygen concentration than air. In Example(s), the firing is carried out at 870xc2x0 C. Further, JP-B-6-30297 to the present Applicant describes the following. When Li2O and an oxide of a tetravalent metal (M4) (in which M4 stands for at least one metal selected from titanium, tin or germanium) are added to a main composition containing iron oxide and an oxide of a divalent metal (M2) (in which M2 stands for nickel and/or copper or a combination of nickel and/or copper with zinc), the firing can be carried out at a low temperature, a temperature of 950xc2x0 C. or lower.
However, nothing has been so far proposed concerning the method of enabling the firing at a low temperature without controlling any one of an atmosphere and a composition.
It is an object of the present invention to provide a method for the preparation of a soft ferrite magnetic powder, which method enables the production of an Nixe2x80x94Cuxe2x80x94Zn ferrite powder having excellent sinterability at a low temperature, and a method for the production of a laminated chip inductor, in which the above ferrite powder is used so that the firing can be carried out at a low temperature to produce a laminated chip inductor.
The above objects are achieved by the present invention recited in the following constitutions.
(1) A method for the preparation of a soft magnetic ferrite powder composed of Fe, Ni, Cu and Zn as main components,
the method comprising the step of allowing an organic additive to be present in a slurry containing a calcined product of a starting powder and water,
wherein the organic additive is an organic compound having a hydroxyl group and a carboxyl group or a neutralized salt thereof or lactone thereof, or the organic additive is an organic compound having a hydroxymethylcarbonyl group, an organic compound having an enol type hydroxyl group dissociable as an acid or a neutralized salt thereof.
(2) A method according to the above (1), wherein the organic additive is added in an amount of 0.05 to 3.0% by weight based on the calcined product.
(3) A method according to the above (1) or (2), wherein the slurry contains Fe ion and Cu ion derived from said calcined product and has an Fe ion and Cu ion total content of 0.005 to 2% by weight based on the calcined product.
(4) A method according to any one of the above (1) to (3), wherein the organic compound having a hydroxyl group and a carboxyl group is gluconic acid or citric acid.
(5) A method according to any one of the above (1) to (3), wherein the organic compound having an enol type hydroxyl group dissociable as an acid is ascorbic acid.
(6) A method according to any one of the above (1) to (5), wherein the slurry contains ammonia. (7) A method for the production of a laminated chip inductor, which comprises the step of forming a magnetic layer from the soft magnetic ferrite powder produce by any one of the methods recited in the above (1) to (6).